Gas discharge lamps typically contain, in addition to an inert gas, a small quantity of metal salt fillers such as metal halide fillers. Generally, metals such as thallium, sodium, indium and the like can be filled in the form of metal salts in a high pressure mercury discharge lamp to improve light emission efficiency, color rendition and the like. These metal salts have a strong tendency to absorb moisture when exposed to the atmosphere. If, during the manufacture of gas discharge lamps, the moisture is inadvertently admitted into the lamp envelopes, the characteristics of the lamps will be greatly impaired. For example thallium iodide will liquify after exposure to the atmosphere for less than one hour. The weight of thallium iodide will also increase upon exposure to the atmosphere (1% alter three minutes), indicating the great tendency for thallium iodide to absorb moisture. Such a contamination not only causes the discharge lamp to blackened, it also results in an increase in the starting voltage. For example, it has been reported in the literature (Japanese Patent Publication 43-17797) that if the discharge lamp was filled with contaminated thallium iodide, the discharge voltage increased to 350 V; whereas, the consequence was even worse for contaminated sodium iodide. In the latter case, the discharge voltage was increased to 450 V. Therefore, it is critically important during the manufacture of metal halide lamps to thoroughly purge the lamp envelope before charging the metal halide fillers to thus prevent the metal halide fillers from absorbing moisture or to keep them from being exposed to the atmosphere.
A conventional prior art apparatus for manufacturing discharge lamps is described in U.S. Pat. No. 3,572,877, in which it is disclosed a glass envelope mounted with electrodes and adapted to form a discharge lamp is connected to one end of an evacuating pipe system made of glass via a first chipoff portion. Two paced apart branch pipes are connected to the evacuating pipe system. The first branch pipe contains the metal halide filler and the second branch pipe contains mercury. The other end of the evacuating pipe system is connected to a vacuum pump and to a source of inert gas to be filled in via a second chipoff portion. The entire system is first evacuated to a high vacuum by the vacuum pump. After the inert gas is admitted under a desired pressure into the envelope, the first chipoff portion is sealed off and then the envelope and the two branch pipes are separated from the remainder of the system. The separated portions, the envelope and the two branch pipes, are then slanted to allow the metal halide filler and mercury contained in the branch pipe to be transferred to the lamp envelope. Thereafter, the second chipoff portion is sealed off and the envelope is separated from the evacuating system, thus completing the manufacture of a metal halide discharge lamp. The content disclosed in the '877 patent is expressly incorporated herein by reference.
The conventional apparatus has several drawbacks, for example: (1) since the metal halide filler and mercury are contained in the same evacuating system, it is difficult to achieve high vacuum; (2) the metal halide filler tends to decompose thus causing non-uniform quality; (3) deposited moisture is difficult to remove; and (4) provisions of the branch pipes for exhausting of each discharge lamp complicates the manufacturing process as well as the construction of the evacuating system.
A number of improvements have been disclosed in the prior art references. For example, the '877 patent disclosed a manifold connected to both the lamp envelope and the vacuum pump and the inner gas supply, and the use of a capsule which contains the metal halide filler. The capsule is carded via a spindle, which is inserted into the manifold after the purging steps to introduce the metal halide filler to the lamp envelope. This method is cumbersome. Furthermore, because there is no provision to isolate the metal halide filler after the purging step, the spindle must be inserted into the manifold against the flow of the inert gas, or the entire operation must be conducted in a dry box environment. Other improved apparatuses or methods, such as those disclosed in U.S. Pat. No. 4,993,981, Japanese Patent Publications Nos. 40-19548, 43-17787, 43-17797, and 46-19390, all suffer similar problems.
The need to use a dry box for the entire duration of the manufacturing steps, including the purging step, complicates the manufacturing process and increases the manufacturing cost. Furthermore, it is difficult to achieve the desired degree of vacuum in a dry box when the dry box has to accommodate all the piping and valving needs, and the manufacturing operation cannot be easily expanded once it is constructed. Yet furthermore, with the methods and apparatuses disclosed in the prior art, only one discharge tube can be produced per operation; this does not indicate an favorable efficiency.